Squirrels belong to Sciuridae family which are small to medium-size rodents familiar to almost everyone around the world, except Australia. There are more than 200 species of squirrels in the Sciuridae family and in general they can be divided into 3 types: tree squirrels, ground squirrels and flying squirrels. Squirrels, like other rodents, have four front teeth that grow continuously so they don't wear down from the constant gnawing.

Tree squirrels are the most commonly recognized, as they are mostly active during the day. In urban environment, they are often the only wild mammals that most people ever see.Tree squirrels include over a hundred primarily tree-dwelling species, examples include:

Grey squirrel (Sciurus carolinensis)

Asian/European red squirrel (Sciurus vulgaris)

Fox squirrel (Sciurus niger)

American red squirrel (Tamiasciurus hudsonicus)

Grey squirrels (Sciurus carolinensis)

Range & habitat
Grey squirrels inhabit eastern United States (west of Mississippi and north to Canada) and introductions and range expansion have brought them also to western states other parts of Canada as well as in Italy, Ireland and the UK.
In the UK grey squirrel is now a widespread and well established species, common throughout central and southern England, Wales and the central lowlands of Scotland.
In the wild grey squirrels live in forests, with preference for mature, dense, broadleaved woodland areas of continuous coverage of more than 40 hectares.
Oak-hickory hardwood forests with rich understory vegetation that provide diverse and abundant food sources and favorable shelters are preferred over coniferous forests.
They also occur in urban environment - in parks, gardens and woodlands.

Dreys
Grey squirrels build their nests, called dreys, in the forks of trees or on large tree branches. The dreys are spherical, about 30 – 60 cm in diameter and consist mainly of dry leaves and twigs, usually lined with moss plants, thistledown, dried grass, and feathers, which provide insulation of the den to reduce heat loss.
In addition to constructing their dreys, they may occasionally inhabit a permanent tree den hollowed out in the trunk or a large branch of a tree or take shelter within abandoned bird nests.
During cold weather squirrels may share a drey to stay warm.

Physical characteristics
Grey squirrels show no sexual dimorphism in size or coloration. As suggested by their name, grey squirrels' fur is predominantly grey but it can vary from dark to pale grey and it can also have touches of brownish colour. They have white underside and pale grey to white ears.
Their large bushy tail is white to pale grey, also with occasional brownish hues and plays an important part in balancing when squirrels jump great distances from tree to tree and is also used for signalling a danger.
They have 22 teeth. Their four front teeth, like in all rodents, grow continuously so they don't wear down from the constant gnawing.
Head and body length: 23-30 cm
Tail length: 15-25 cm
Adult weight: 400-750 g

Diet
Grey squirrels are predominantly vegetarian and their primary food range includes a wide variety of berries, tree seeds, buds, flowers, seeds and acorns, walnuts and other nuts, some types of fungi.
On very rare occasions in times of food deficiency they look for alternative food sources and can eat insects, frogs, bird eggs or tree sap. They also gnaw on bones (e.g. antlers) – likely as a source of minerals to supplement their normal diet.

Reproduction
Grey squirrels breed twice a year: in winter in December to February/March and in summer between June and July. Male squirrels are reproductively mature between one and two years of age. Females are fertile around one year of age and younger ones usually have a single litter per year in the spring. Depending on climate and availability of food, more experienced females may breed again in summer.
Each litter usually contains two to four kittens but up to 8 young are possible.
Gestation lasts around 44 days.
The young are born blind, deaf and naked except for vibrissae (small hairs around the nose and mouth that are used for touch, much like the whiskers of a cat). They weigh 13-18 g.
Weaning occurs at around 10 weeks and the young begin to leave the nest after 12 weeks. Some may wean up to six weeks later in the wild and autumn born young often winter with their mother.

European red squirrel (Sciurus vulgaris)

Range & habitat
Red squirrels range spans throughout Europe and into northern Asia and Siberia where they live at altitudes of up to 2,000 metres.
In the UK the red squirrel currently resides in most of mainland Scotland, Brownsea Island and the Isle of Wight and it is also found in some habitats in parts of Wales, Northern Ireland and England.
In the wild red squirrels occupy forests with large, mature trees. They prefer dense coniferous woodland with Norway spruce, Scots pine and Siberian pine and they need a minimum area of 200 hectares of unfragmented forests to thrive.
Even in smaller coniferous forests, red squirrels do significantly better than in deciduous or mixed forest where food sources are not so suitable for them. Planting large, dense coniferous woodlands is an important factor in helping red squirrels populations grow.
They are also found in gardens and parks in urban environment.

Dreys
Red squirrels build nests, called dreys, placed high in the branches or in tree cavities. The dreys consists of an outer frame of twigs and sticks, lined with moss, dry leaves, soft hair and grass. Squirrels can have several nests at a time that they use as shelter.

Physical characteristics
Red squirrels fur colour can vary depending on the time of year and location, ranging from black through chestnut to red. In the UK red coats are the most common. They have white-cream underside. Long ear tufts are the distinguishing feature of red squirrels
Their long furry tail moults once a year, like ear tufts, and the rest of the coat is moulted twice a year – for winter and for summer.
Their four front teeth, like in all rodents, grow continuously so they don't wear down from the constant gnawing.
Head and body length: 18-24 cm
Tail length: 15-20 cm
Adult weight: 250-350 g

Diet
The primary diet of red squirrels consists mainly of coniferous tree seeds, buds, flowers, shoots, nuts, berries and other fruit and fungi. Although predominantly vegetarian red squirrels can, in times of hunger, be seen eating bird eggs. In such times of lower food availability they can be also peeling the bark off conifers and licking the trees' sap.

Reproduction
Red squirrels breeding seasons are in winter / early spring during February and March and in summer between June and July. Young red squirrels are sexually mature within a year however typically a female will produce her first litter in her second year.
Normally, three to four young are born in each litter, but possible litter size is up to six.
The gestation period is approximately 38 days.
Newborns are hairless, blind and deaf, weighing 10-15 g. The fur grows around three – four weeks at which time also their ears and eyes begin to open.
The young are weaned at 8-10 weeks. They start eating solid foods around day 45 and can begin to leave the nest at this point but are not fully independent until 3-4 months.

Fox squirrel (Sciurus niger)

Range & habitat
Fox squirrels live in North America, with native range from the eastern United States, north into south Canada, and to mid-west US. They have been introduced to California, Oregon and other western states. Fox squirrels live mostly in forest patches of 40 hectares or les of open forest with with an open understory.
Eastern fox squirrels also commonly occupy forest edge habitat and can also be found in urban neighborhoods with trees.

Dreys
Fox squirrels find shelter in leaf nests or tree dens (preferred during the winter and when raising young). They may have more than one nest at their disposal. They construct leaf nests in the summer in forks of deciduous trees high in the canopy. Bird nests can sometimes be used. They generally use natural cavities as dens which are generally 15 cm wide and around 40 cm deep, with circular openings of about 8 cm.

Physical characteristics
Their upper body is is brown-grey to brown-yellow and in various areas other colouration patterns are seen, up to dark brown and black coats. Their underside is typically brownish-orange.
Fox squirrels have sharp claws, and very well developed vision, hearing and smell. They have vibrissae, thick hairs or whiskers that are used as touch receptors to sense the environment.
Total body length: 25-43 cm
Tail length: 20-33 cm
Adult weight: 500-1000 g

Diet
Diet of fox squirrels is mostly vegetarian: tree seeds (oak, hickory, walnut, pine), tree buds, bulbs, roots and fungi. The can also eat crops such as corn, soybeans, oats, wheat, and fruit. Sometimes they can include insects, bird eggs in their diet although these are opportunistic food, not primary.

Reproduction
Female eastern fox squirrels come into estrus in mid-December or early January then again in June. They normally produce two litters a year, however, yearling females may only produce one.
Each litter usually contains three kittens but can vary according to season and food conditions.
Gestation lasts around 44 to 45 days.
At birth, young fox squirrels are blind, without fur and helpless. Eyes open at 4 to 5 weeks of age and ears open at 6 weeks. They are weaned between 12 and 14 weeks but may not be self-supporting until 16 weeks.

American red squirrel (Tamiasciurus hudsonicus)

Range & habitat
Red squirrels occur in almost all of the northern half of North America. Their range extends south along the Rocky Mountains to Arizona and New Mexico and east all the way to Quebec, Canada. Red squirrels can also be found in Alaska. They use forest habitat and prefer to live in shady forests with tall pine trees and mushrooms. They can also live in other kinds of forests as well as around human homes if there are cool pine forests nearby. They live in geographical areas with seasons and also in places that are cold all year round. In most places they live and especially in pine forests, red squirrels claim and defend territories which can be in the range from 2400 to 48000 square meters in size. They stockpile food supplies and don't allow others in.

Dreys
American red squirrels build their nests in the trees and an individual usually has several nests. The nests are made of grass, moss, parts of plants, shredded bark, feathers, or fur. Winter nests are weather-tight and constructed in the densest foliage of a tree. They can also use holes in a tree trunk as shelter.

Physical characteristics
Red squirrel fur colour depends on the region and season. The upper parts are usually brownish or reddish or olive grey, but usually has a reddish or brownish band along the middle. They have a white underside with dark coloured lines which are easiest to see in summer. The tail, smaller and flatter than other tree squirrels, can be yellowish-grey or rusty red and is often edged with white. There are white bands encircling their large, black eyes.
Head and body length: 16-23 cm
Tail length: 9-15 cm
Adult weight: 140-310 g

Diet
Red squirrels eat mostly seeds of pine trees but are flexible depending on seeds availability. They eat acorns, hazelnuts, other seeds of deciduous trees, berries, fruit tree buds and flowers, fleshy fruits, tree sap, bark, different kinds of mushrooms, insects, and even bird eggs. Red squirrels hoard food, so they take pine tree cones and store them somewhere moist and cool. They store up enough food to last one or two seasons. The same storage spots are often used by several generations of squirrels.

Reproduction
American red squirrels breed once or twice a year, depending on where they live. Where it is colder, they usually breed once a year. They mate in early spring from March to May and again in August to early September.
Normally, four young are born in each litter, but possible litter size is up to eight.
The gestation period is approximately 33-35 days.
Newborns weight around 7 g and have no hair except for whiskers and some soft hairs on their chins. They open their eyes after 26 to 35 days, and they have all of their fur after 40 days.
The young are weaned at 70 days. They can begin to leave the nest around 7 weeks. Young red squirrels are full size and have all of their teeth by the time they are 125 days old.